Investigations are in progress to determine whether the malonyl CoA required for fatty acid biosynthesis in propionic acid bacteria arises from the reaction catalyzed by acetyl CoA carboxylase or by the reaction mediated by oxaloacetate transcarboxylase. The activity of acetyl CoA carboxylase is being determined in cell-free extracts (unfractionated and fractionated by the addition of ammonium sulfate) prepared from P. shermanii. Experiments are in progress to determine conditions (e.g. growth on media containing different carbon source) inducing acetyl CoA carboxylase in these bacteria. Furthermore, the role of malonyl CoA decarboxylase in regulating bacterial lipogenesis is being investigated.